This invention pertains to electrodeless discharge lamps.
Discharge lamps and particularly electrodeless discharge lamps which contain a condensable fill are known. When the lamp is not operating and cold, the condensable portion of the fill is condensed on the inside of the lamp envelope. These lamps usually also contain a gas which remains gaseous even at low temperatures. This gas facilitates starting as will be described below, and it may also serve the purpose of affecting the performance of the plasma by changing the thermal conductivity of the plasma.
Capacitively coupled, inductively coupled, and microwave excited varieties of electrodeless lamps are known. All of these lamp have in common that the power is supplied to the lamps, not through electrodes which penetrate the bulb, but rather by being subject to an externally produced electromagnetic oscillation. The variation of the pattern of the electromagnetic field, depends on the structure and operation of the external source of the electromagnetic field. Generally there are some areas of the bulb which are subject to higher electromagnetic field, at least during start up.
The starting process of discharge lamps with condensable fill and starting gas has several stages. At first the electromagnetic field is applied, then some minute ionization occurs in the bulb, perhaps by the incidence of a gamma ray from outer space, or as a result of photoelectrons being emitted from the envelope or condensed fill by the action of irradiation from an auxiliary source of ultraviolet light, or by some other agency. The electromagnetic field energizes the electrons and an avalanche breakdown occurs which leads to ionization of the whole starting gas (to some extent e.g. first or second ionization) to form a plasma therefrom. This initial plasma will be relatively low power density and may have a variation in intensity over the interior of the bulb that is different from that of the steady state plasma. The starting gas plasma heats the bulb envelope and thereby causes the evaporation of the condensable fill which is in turn ionized to partake of the discharge. As the condensable fill evaporates, the discharge becomes higher power until all the fill is vaporized and the power reaches its steady state value. The change in power absorbed by the bulb changes, because the impedance of the bulb changes as the condensable fill evaporates and the pressure in the bulb increases.
Upon turning off the power to the lamp, the condensable fill condenses in the area of the interior of the lamp which cools off fastest. This portion may be the area subject to the most forced external cooling e.g. under the cooling air jet, or the area which runs coolest at full power operation.
As discussed above, the starting gas plasma has some variation in intensity over the interior of the bulb. If the starting gas plasma is not very intense in the area of the bulb where the condensable fill condenses it takes a long time to evaporate the condensable fill and thus start the bulb. It may even be impossible, and even if it can be done the interval varies from one start up to the next, i.e., it is not repeatable.
Linear microwave electrodeless lamps made by the assignee of the instant invention direct cooling air and radiate microwave power toward the bulb from the same side. Thus, upon turning off the power, the fill condenses on the side of the bulb which receives power upon restarting the lamp.